1. Field of the Invention
The present invention is directed generally to output circuits and, more particularly, to output drivers having the ability to control the rise and fall times of the signals output therefrom.
2. Description of the Background
Drive circuits, or drivers as they are commonly called, are used in a variety of applications. Typically, the driver acts as an interface between a logic circuit, or other circuitry where signals are being manipulated at very low levels and circuits or loads which require high signal levels or large current levels. For example, drivers are found in various types of memory devices as the interface between data pads and the internals of the memory device. In such an environment, the driver is typically used to drive the data pad to a first voltage to represent a logic level 1 and a second voltage to represent a logic level 0. The driver typically must source sufficient current to enable signals available at the data pads to travel significant lengths along buses or to be sensed by other types of loads.
Prior art drive circuits typically utilize a pair of output drive transistors designed to operate in a complementary fashion. For example, the data pad may be connected to the first voltage through an N-type transistor and connected to the second voltage through a P-type transistor. When one of the transistors is on, the other transistor is off. In modern circuits, both transistors may be off to allow the data pad to receive data.
When designing drivers, it is necessary to insure that the transistors turn off as quickly as possible so that the situation does not exist wherein both transistors are on. Additionally, because of the high operating speeds of integrated circuits, it is necessary for the transistors to rapidly change state. It is also desirable to produce a driver which provides an output signal that is substantially symmetrical with respect to its leading edge and falling edge. However, due to process variations, the signal path servicing the N-type transistor may differ from the signal path servicing the P-type transistor such that the resulting output signal is not symmetric. Additionally, timing specifications for various applications may require adjustment of the leading or falling edge of the output signal. Accordingly, the need exists for a driver capable of compensating for process variations and for providing flexibility with respect to the timing of the output signal produced by the driver.